1. Field of the Invention
The present invention generally relates to the field of electrical signal processing circuitry, and more particularly to the use of such circuitry for controlling internal combustion engine parameters such as the spark advance in an ignition system.
2. Description of the Prior Art
It has been recognized that the present day mechanical ignition systems for automobiles and similar vehicles cannot meet the requirement for reliably controlling the spark timing, dwell and the proper spark advance of an internal combustion engine over the estimated lifetime of the engine. Generally, most prior art electronic ignition systems utilize a crankshaft position sensor for synchronizing developed electronic control signals to predetermined positions of the engine crankshaft. However, the accuracy of prior art electronic ignition systems is critically dependent upon the duty cycle of the crankshaft position sensor signal. Since the duty cycle of the sensor signal (the ratio of one logic state produced by the sensor to the period of the sensor signal) may vary substantially for various configurations of sensors and under certain extreme engine conditions, most prior art electronic ignition systems have been unable to utilize the sensor signal to accurately control the dwell and spark timing of the engine with the precision which is desired.
One system describes a constant duty cycle monostable multivibrator circuit to produce a simulation of the pulses generated by a conventional point contact ignition system. The time during which the contact points would be held open is made equivalent to the time period of the output pulse generated by the circuit in its unstable state. The ratio of the times for the stable and unstable states or duty cycle of the monostable is a constant although the magnitude of each time individually varies inversely with engine speed.
U.S. Pat. No. 4,170,209, issued Oct. 9, 1979, to Petrie et al. titled "Ignition Dwell Circuit for an Internal Combustion Engine" and assigned to the assignee of the present application shows a circuit especially useful for producing output pulses which occur a fixed time before a predetermined rotational position of an engine's crankshaft. The circuit includes a position sensor triggered dual slope integration circuit producing signals which provides an accurate division of the rotational motion. A clamping circuit generates a corresponding signal having a peak magnitude which corresponds to a first predetermined reference level. A comparator produces a pulse when the clamping signal corresponds to a second reference level which occurs at the fixed time before the rotational position is reached.